10 Years of O&M Experience with Full-Scale Biogas Desulphurization in Municipal Wastewater Treatment Plants
For over a decade, WATERLEAU MAROCCO has been at the forefront of operating and maintaining biogas desulphurisation units in municipal wastewater treatment plants. These systems play a critical role in maximizing biogas quality and renewable energy production. However, with the rise of the European biomethane market, new challenges arise.
Introduction
For over 15 years, WATERLEAU MAROC has been responsible for the operation and maintenance (O&M) of municipal wastewater treatment plants (WWTPs), including anaerobic digestion of primary and secondary sludge to produce biogas, followed by biogas desulphurization and cogeneration to supply renewable electricity.
The transformation of sludge into biogas plays a crucial role in the overall sustainability of municipal WWTPs. In fact, up to 60% of a plant’s electrical consumption can be covered by the biogas generated on site.
WATERLEAU MAROC currently operates five large-scale municipal WWTPs, each equipped with full biogas lines—from anaerobic digestion to biogas desulphurization and combined heat and power (CHP) generation.
This blog article focuses specifically on our operational experience with biogas desulphurization units over the past decade and reflects on how these insights relate to the current and future challenges of the European biomethane market.
Types of Biogas Desulphurization Units
In its municipal wastewater treatment plants, WATERLEAU has predominantly implemented acid biological trickling filters from various suppliers.
These filters operate independently from the main WWTP processes and are specifically designed to remove hydrogen sulphide (H₂S) from biogas prior to its use in cogeneration units or further upgrading.
Working Principle of Acid Biological Trickling Filters with Air Injection
Acid biological trickling filters rely on a specific group of aerobic, sulphur-oxidising bacteria that convert hydrogen sulphide (H₂S) into sulphuric acid (H₂SO₄). This is achieved through a two-step oxidation process, with air injected into the filter to provide the necessary oxygen:
- Partial oxidation:
H₂S (sulphur in oxidation state -2) + O₂ → S⁰ (elemental sulphur) - Complete oxidation:
S⁰ + O₂ → SO₄²⁻ (sulphate, sulphur in oxidation state +6)
The result is an acidic environment where H₂S is converted into sulphate and protons (2H⁺), contributing to the low pH conditions inside the filter. The system requires precise oxygen management, regular water dosing, and nutrient supply to maintain biological activity and ensure optimal H₂S removal efficiency.
Conclusions from 10 Years of Operation & Maintenance (O&M)
1. Influence of oxygen concentration in the gas phase:
To perform optimally, acid biological trickling filters typically require a high oxygen concentration in the biogas stream—above 1 vol%, with ideal conditions often around 1.5 vol% O₂.
WATERLEAU MAROC’s 10 years of experience across four operational units confirms this requirement: all systems consistently operate above 1 vol% O₂, generally maintaining around 1.5 vol%.
This high oxygen level is essential to drive the two-step oxidation reaction of H₂S to sulphate (SO₄²⁻). At lower oxygen levels, the process favors the formation of elemental sulphur (S₀), which can lead to clogging and reduced performance.
2. Clogging Tendency, Maintenance Interventions, and Cost
All four systems experienced clogging over time, requiring frequent and labor-intensive maintenance:
- Between cleanings, H₂S removal efficiency gradually decreases due to clogging, creating preferential flow paths and dead zones in the filter media.
- While the reduced removal efficiency (from ~200 ppm to 400–500 ppm H₂S) is acceptable for cogeneration units, it results in higher maintenance costs for biogas engines.
3. Maintenance strategies vary by site:
- Routine cleaning every 6–8 weeks, involving draining the filter, removing accumulated biomass, reseeding, and restarting. This takes ~4 days per cycle and results in about 29 days of downtime annually, or nearly 10%.
- Deep cleaning every 6 months when intervals are longer. Clogging becomes more severe, requiring 7 days of biomass removal and 1 week of reassembly and startup—again leading to around 10% annual downtime.
- New cleaning systems are being tested to reduce downtime and improve efficiency.
While this is manageable for biogas-to-power systems, it presents a challenge for biomethane applications, where H₂S levels must be reduced to < 2–3 ppm.
Therefore, in biomethane projects, acid biological trickling filters must be followed by an activated carbon polishing step. Given the risk of high H₂S slip (up to 400 ppm), the OPEX for activated carbon must be carefully calculated.
4. Replacement Costs of Internal Components
The O&M experience also highlights the need for regular replacement of internal components, particularly:
- The biomass carrier material
- The cleaning water distribution systems
These components are continuously exposed to low pH environments and oxygen, accelerating wear and tear. Replacement is typically required annually or biennially.
Considering these internals are stainless steel, these costs are logical and expected, but they must be factored into long-term OPEX.
5. Dedicated Follow-Up, Nutrients, and Water Quality
Effective operation of acid biological trickling filters requires:
- Consistent dosing of chemical nutrients, which increases OPEX but helps limit clogging.
- The use of soft water for feed and bleed, which also helps to control scaling and clogging.
- Daily monitoring by skilled operators—typically around 1 hour per day.
Challenges for Acid Biological Trickling Filters in the Rapidly Growing Biomethane Market
While acid biological trickling filters have proven effective in WWTPs with biogas-to-energy applications, their use in biomethane upgrading chains presents several challenges:
- Incompatibility with low oxygen requirements
Biomethane injection into the grid requires very low oxygen concentrations, while acid biological trickling filters operate best at high oxygen levels (>1–1.5 vol% O₂) to ensure optimal H₂S removal and prevent clogging.
This fundamental mismatch makes it difficult to balance safe biomethane standards with efficient filter performance. - Reduced desulphurisation efficiency under low O₂
Operating these filters at lower oxygen concentrations—as would be required upstream of a biomethane upgrading system—leads to incomplete H₂S removal, often resulting in outlet concentrations of 400 ppm or more. This significantly increases the load and OPEX of downstream activated carbon polishing (needed to bring H₂S down to <2–3 ppm). - High downtime and maintenance expectations
Given the tendency to clog under fluctuating loads, at least 10% operational downtime should be factored into financial and operational planning, along with high maintenance effort and replacement costs.
BIOTIM® Scrubber: A Reliable Alternative for Biomethane Applications
The BIOTIM® scrubber developed by Waterleau offers a robust and biomethane-ready alternative to acid biological trickling filters:
- Operates at very low oxygen concentrations, fully compatible with biomethane grid standards.
- High operational uptime (>99.5%), with predictable performance.
- Automated cleaning system requires less than 10 hours and full restart within 10 minutes, every 4 months.
- Minimal operator involvement: typically, less than 1 hour per week.
- High flexibility to handle rapid fluctuations in H₂S load and concentration without compromising performance.
Conclusion
Over the past decade, acid biological trickling filters have proven effective in municipal WWTPs for biogas desulphurization in cogeneration setups. However, their incompatibility with low-oxygen requirements, frequent clogging, and intensive maintenance needs make them less suitable for biomethane applications.
As Europe accelerates the rollout of biomethane into its energy mix, technologies like the BIOTIM® scrubber offer a future-proof solution: flexible, low-maintenance, oxygen-compliant, and grid-ready. For operators looking to transition from biogas to biomethane, it represents a reliable, high-performance choice with long-term operational and economic advantages.
Discover the best biogas desulphurisation solution for your needs
Are you in search of a durable, adaptable, and user-friendly solution for biogas and extraction gas treatment?
Reach out to us today. We are eager to share our expertise in biogas purification and introduce you to our BIOTIM® scrubber technology.
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